The Planet Mercury -- in astronomy, nearest planet to the sun, at a mean distance of 36 million mi (58 million km); its period of revolution is 88 days.
Mercury passes through phases similar to those of the moon as it completes each revolution about the sun, although the visible disk varies in size with respect to its distance from the earth. Because its greatest elongation is 28, it is seen only for a short time after sunset or before sunrise.
Since observation of Mercury is...

A pivoted catch designed to fall into a notch on a ratchet wheel so as to allow movement in only one direction (e.g. on a windlass or in a clock mechanism), or alternatively to move the wheel in one direction.